Process and apparatus for production of carbon monoxide by partial condensation

ABSTRACT

A process for separating a mixture of hydrogen, methane and carbon monoxide by cryogenic distillation. The mixture is cooled in a first heat exchanger and is partially condensed in at least one second heat exchanger which is a bottom reboiler of a first or second column of the column system and at least one portion of the liquid produced by the partial condensation is sent to an intermediate level of the first column.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority under 35 U.S.C. § 119 (a) and (b) to French Patent Application No, 1909300, filed Aug. 20, 2019, the entire contents of which are incorporated herein by reference.

BACKGROUND

The present invention relates to a process and to an apparatus for the production of carbon monoxide by partial condensation.

Units for the production of carbon monoxide and hydrogen can be separated into two parts:

Generation of Synthesis Gas

(Mixture containing H₂, CO, CH₄, CO₂, Ar, and N₂, essentially). Among the various industrial routes for the production of synthesis gas, that based on coal gasification appears to be increasingly expanding, in particular in countries rich in deposits of coal, such as China. The process for the partial oxidation of natural gas can also prove to be advantageous for the production of CO, alone or with low H₂/CO production ratios. Another route is steam reforming.

Purification of the Synthesis Gas.

The following are found:

-   -   a unit for scrubbing with a liquid solvent in order to remove         most of the acid gases present in the synthesis gas;     -   a unit for purification on a bed of adsorbents;     -   a unit for cryogenic separation, referred to as a cold box, for         producing CO.

In the case where the synthesis gas is produced from an entrained bed or fluidized bed coal gasification, the cold box process is partial condensation. For MEG, TDI/MDI or PC applications for example, it is necessary to include a CH4 separation column in the cold box.

In the diagram from EP 0677483, the reboiling energies of the two flash separation and CO/CH4 separation columns are supplied by cooling of the synthesis gas in the main exchanger and by the cycle. The synthesis gas is cooled in its entirety up to the cold end of the heat exchanger before re-entering the separator vessel for separating the hydrogen.

The drawbacks are that:

-   -   The reboiling of the two columns in the exchange line does not         enable good regulation of the reboiling energies.     -   The energy consumption of the refrigeration cycle is high since         the synthesis gas is cooled in its entirety up to the cold end.         There is no vessel for intermediate condensation of the         synthesis gas in order to extract the condensed liquid therefrom         and avoid subcooling it.

WO 2018039313 describes independent reboilers of the two columns with the cooling of the synthesis gas but the whole of the synthesis gas (outgoing liquid and gas leaving the reboilers) is cooled up to the cold end of the exchange line according to FIGS. 3, 4 and 5. In FIGS. 1 and 2, the two reboilers are in series, the whole of the output from the reboiler of the flash column is sent to the separator vessel shared with the cold end. The outgoing liquid leaving the reboiler of the CO/CH4 column is subcooled in the exchange line.

The drawbacks are that:

-   -   If the two reboilers are in series, this creates instabilities         in the reboiling energy of the stripping column when the         temperature of the outlet from the first reboiler varies as a         function of compositional disturbances of the incoming synthesis         gas.     -   The output from the second reboiler is sent directly to the         high-pressure synthesis gas vessel. In order for the output from         the second reboiler to be at a temperature close to the         high-pressure synthesis gas vessel, it is necessary to lower the         pressure of the stripping column, which has the effect of         increasing the amount of CO in the overhead gas. The CO yield is         thus reduced if the flash gas is not recycled, or else the         compression energy of the recycling of the overhead gas is         increased.     -   The CO/CH4 column is operated at low pressure following the         pressure drop of the stripping column, which increases the         compression energy of the CO output from this column.     -   The energy of the refrigeration cycle is high since all the         synthesis gas is cooled up to the cold end, the liquid at the         outlet of the vessel for partial condensation of the synthesis         gas at the outlet of the reboilers is subcooled and therefore         consumes refrigeration cycle.

In U.S. Pat. No. 6,266,976, the reboiling energies of the two columns are provided by cooling of the synthesis gas and of the cycle in the exchange line. The liquid from the first vessel for partial condensation of the synthesis gas output from the first exchanger of the exchange line is sent to the stripping column but as reflux and not at an intermediate level (fluid 146).

A portion of the liquid from the second vessel for partial condensation of the synthesis gas is sent as reflux of the stripping column but after having been reheated in the exchange line.

The drawbacks are:

-   -   The reboiling of the two columns in the exchange line does not         enable good control of the reboiling energies,     -   High CO content in the flash gas at the stripping column         overhead outlet which leads to an increase in the energy for         recycling the flash separation gas or else loss of CO yield if         the flash separation gas is not recycled.

SUMMARY

According to one subject of the invention, a process is provided for separating a mixture of hydrogen, methane and carbon monoxide by cryogenic distillation in a column system comprising a first column and a second column, in which process:

i. the mixture is cooled in a first heat exchanger and at least one portion thereof is partially condensed in at least one second heat exchanger which is a bottom reboiler of the second column of the column system and at least one portion of the liquid produced by the partial condensation or a liquid derived from this liquid by at least one step of partial condensation is sent to an intermediate level of the first column, only the at least one portion of the mixture and bottom liquid from the second column exchanging heat in the second heat exchanger, the vaporized bottom liquid being sent back to the second column from which it originates,

ii. a hydrogen-enriched overhead gas is withdrawn from the first column and is reheated in the first heat exchanger,

iii. a liquid enriched in carbon monoxide and in methane is withdrawn from the bottom of the first column and it is sent to at least one intermediate level of the second column,

iv. a methane-rich liquid is withdrawn from the bottom of the second column and a fluid rich in carbon monoxide is withdrawn from the top of the second column, and

v. refrigeration is produced to keep the process cold by means of a refrigeration cycle.

According to other optional aspects:

-   -   a portion of the cooled mixture is sent to a reboiler of the         first column;     -   at least one portion of the cooled mixture is sent to be cooled         to the second exchanger which is a reboiler of the second column         and then the at least one portion of the cooled mixture is sent         to a third exchanger which is a reboiler of the first column;     -   at least one portion of the cooled mixture is sent to be         partially condensed in the reboiler of the second column to         produce a gas and a liquid, then the gas is sent to a third         exchanger which is a reboiler of the first column and at least         one portion of the liquid produced by at least partial         condensation of the gas in the third exchanger is sent to the         intermediate point of the first column;     -   a condensed portion, produced following at least one step of         partial condensation of the mixture, is sent to the top of the         first column of the column system;     -   the first column has a bottom reboiler and has no overhead         condenser;     -   the first column operates at between 13 and 16 bar abs;     -   the second column operates at a pressure above 6 bar abs;     -   the second column has a bottom reboiler and an overhead         condenser;     -   the overhead gas from the first column contains less than 35 mol         %; or even less than 25 mol %, of carbon monoxide;     -   the overhead gas from the first column contains more than 60 mol         %, or even more than 75 mol %, of hydrogen;     -   the mixture is cooled in the first heat exchanger (3,9) to an         intermediate temperature thereof and then leaves the first heat         exchanger before being sent to the reboiler;     -   the fluid rich in carbon monoxide from the top of the second         column and/or the hydrogen-enriched overhead gas from the first         column is (are) cooled in the first heat exchanger from the cold         end of the first heat exchanger;     -   a gas derived by partial condensation of the mixture cooled in         the first heat exchanger is cooled in the coldest part of the         first heat exchanger.

According to another subject of the invention, an apparatus is provided for separating a mixture of hydrogen, methane and carbon monoxide by cryogenic distillation, said apparatus comprising a column system comprising a first column and a second column, a first heat exchanger, a second heat exchanger, means for sending the mixture to be cooled to the first heat exchanger, means for sending the cooled mixture to be partially condensed to at least the second heat exchanger which is a bottom reboiler of the second column of the column system, means for sending to an intermediate level of the first column at least one portion of the liquid produced by the partial condensation or a liquid derived from this liquid by at least one step of partial condensation, only the mixture and bottom liquid from the column exchanging heat in the second heat exchanger, means for sending the vaporized bottom liquid back to the column from which it originates, means for withdrawing a hydrogen-enriched overhead gas from the first column, means for sending the overhead gas to be reheated to the first heat exchanger, means for withdrawing a liquid enriched in carbon monoxide and in methane from the bottom of the first column, means for sending the liquid enriched in carbon monoxide and in methane to at least one intermediate level of the second column, means for withdrawing a methane-rich liquid from the bottom of the second column, means for withdrawing a fluid rich in carbon monoxide from the top of the second column and a refrigeration cycle to produce refrigeration to keep the process cold.

BRIEF DESCRIPTION OF THE DRAWINGS

For a further understanding of the nature and objects for the present invention, reference should be made to the following detailed description, taken in conjunction with the accompanying drawings, in which like elements are given the same or analogous reference numbers and wherein:

FIG. 1 is a schematic representation of a process for separating a gas mixture, in accordance with one embodiment of the present invention . . . .

FIG. 2 is another schematic representation of a process for separating a gas mixture, in accordance with one embodiment of the present invention . . . .

FIG. 3 is another schematic representation of a process for separating a gas mixture, in accordance with one embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 describes a process for separating a gas mixture, such as a synthesis gas, according to the invention. The two reboilers in parallel with a shared vessel at the outlet of the two reboilers, the resulting gas is cooled in the exchange line and the liquid fraction is sent to the intermediate part of the stripping column, A gas mixture 1 containing at least carbon monoxide, hydrogen and methane is cooled in the first section 3 of a heat exchanger that has two sections 3, 9.

The cooled mixture is split into two portions 5, 7. The portion 5 is partially condensed by cooling it in a bottom reboiler 13 of a column K1, which is a stripping column. The portion 7 is partially condensed by cooling it in a bottom reboiler of a column K2, which is a CO/CH4 distillation column. The partially condensed flows are mixed in a phase separator 15.

The gas produced is cooled in the section 9 of the exchanger where it is partially condensed and is sent to a phase separator S2. The gas 63 from the phase separator S2 is reheated in the sections 9, 3. The liquid from the phase separator S2 is sent to the top of the column K1.

The liquid 17 from the phase separator 15 is expanded then mixed with another flow to form a two-phase flow 19. The flow 19 is separated in a phase separator to form a gas 21 and a liquid 23, the two being sent to an intermediate level of the column K1.

A portion 45 of the liquid from the phase separator 15 is cooled in the section 9 of the heat exchanger where it is partially condensed before being sent to a phase separator S1.

The bottom liquid from the phase separator S2 is split into two. One portion 61 is sent as liquid to the top of the column K1. The remainder 43 is expanded and feeds the phase separator S1. The liquid 41 formed in the separator S1 is vaporized in the section 9 to form a gas mixture with the gas from the separator S1 and the flow formed 45 is mixed with the flow 17 from the separator 15.

The overhead gas 21 from the column K1 is reheated in the two sections 9, 3 to form a hydrogen-rich gas 63.

Bottom liquid from the column K1 is vaporized in the reboiler 13 and sent back to the column K1.

The column K1 operates at a pressure between 13 and 16 bar abs and the overhead gas 21 has the following composition:

-   -   CO: 20-35 mol %     -   H2: 65-80%     -   CH4: 0.1%     -   N2: 0.5%     -   Ar: 0.1%

Bottom liquid from the column K1 is split into two to form two expanded liquid flows 27, 31. The expanded flow 31 is sent to an intermediate level of the second column K2 and the expanded flow 27 is partially reheated in the section 9 in order to be sent to an intermediate level of the second column K2 lower than that of the inlet of the flow 31.

A bottom liquid 21 is sent to the reboiler 11, is vaporized and is sent back to the column K2, which operates at a pressure above 6 bar abs.

Another bottom liquid 23 from the column K2 is reheated in the section 3 to form a methane-rich product.

An overhead gas 37 from the column K2 is reheated in the sections 9, 3 to form a carbon monoxide-rich product.

The overhead condenser C of the CO/CH4 column K2 is fed with liquid by a nitrogen cycle, comprising two compressors V1, V2 and with two levels of vaporization at different pressures in the section 9. This cycle is completely independent of remainder of the process and may be replaced by a different refrigeration cycle.

FIG. 2 describes a process for separating a gas mixture according to the invention, similar to that of FIG. 1. The two reboilers 11, 13 are in series. Thus, the mixture 1 cooled in the section 3 solely arrives as the flow 5 in the first reboiler 11, vaporizes the bottom liquid 21 and is partially condensed thereby. The partially condensed mixture is sent to the separator vessel 15 which is at the outlet of the first reboiler 11. The liquid 17 from the vessel 15 is sent to a vessel 65, the liquid 67 of which is sent to an intermediate part of the column K1. The gas 77 from the vessel 15 is partially condensed in the second reboiler 13. A second vessel 65 is at the outlet of the second reboiler 13. The vapor fraction of the second vessel 65 is cooled in the exchange line 9 whilst the liquid fractions 17, 67 of the two vessels 15, 65, at the reboiler outlet respectively are sent to an intermediate part of the column K1.

The solution should be selected from those of the two figures as a function of the pressure of the incoming synthesis gas 1 and the content of CO in the incoming synthesis gas 1.

It will be understood that many additional changes in the details, materials, steps and arrangement of parts, which have been herein described in order to explain the nature of the invention, may be made by those skilled in the art within the principle and scope of the invention as expressed in the appended claims. Thus, the present invention is not intended to be limited to the specific embodiments in the examples given above. 

What is claimed is:
 1. A process for separating a mixture of hydrogen, methane and carbon monoxide by cryogenic distillation in a column system comprising a first column and a second column, comprising: i. cooling the mixture in a first heat exchanger and partially condensing at least one portion thereof in at least one second heat exchanger which is a bottom reboiler of the second column of the column system and at least one portion of the liquid produced by the partial condensation, or a liquid derived therefrom, by at least one step of partial condensation is sent to an intermediate level of the first column, only the at least one portion of the mixture and bottom liquid from the second column exchanging heat in the second heat exchanger, the vaporized bottom liquid being sent back to the column from which it originates, ii. withdrawing a hydrogen-enriched overhead gas from the first column and reheating the hydrogen-enriched overhead gas in the first heat exchanger, iii. withdrawing a liquid enriched in carbon monoxide and in methane from the bottom of the first column and sending it to at least one intermediate level of the second column, iv. withdrawing a methane-rich liquid from the bottom of the second column and withdrawing a fluid rich in carbon monoxide from the top of the second column, and v. producing refrigeration to keep the process cold by means of a refrigeration cycle.
 2. The process according to claim 1, in which a portion of the cooled mixture is sent to the second heat exchanger which is a reboiler of the first column.
 3. The process according to claim 1, in which at least one portion of the cooled mixture is sent to be cooled to the second exchanger which is a reboiler of the second column and then the at least one portion of the cooled mixture is sent to a third exchanger which is a reboiler of the first column.
 4. The process according to claim 1, in which at least one portion of the cooled mixture is sent to be partially condensed in the second exchanger which is a reboiler of the second column to produce a gas and a liquid, then the gas is sent to a third exchanger which is a reboiler of the first column and at least one portion of the liquid produced by at least partial condensation of the gas in the third exchanger is sent to the intermediate point of the first column.
 5. The process according to claim 1, in which a condensed portion produced following at least one step of partial condensation of the mixture, is sent to the top of the first column of the column system.
 6. The process according to claim 1, in which the first column has a bottom reboiler and has no overhead condenser.
 7. The process according to claim 1, in which the mixture is cooled in the first heat exchanger to an intermediate temperature thereof and then leaves the first heat exchanger before being sent to the reboiler.
 8. The process according to claim 1, in which the fluid rich in carbon monoxide from the top of the second column and/or the hydrogen-enriched overhead gas from the first column is (are) cooled in the first heat exchanger from the cold end of the first heat exchanger.
 9. An apparatus for separating a mixture of hydrogen, methane and carbon monoxide by cryogenic distillation, said apparatus comprising a column system comprising a first column and a second column, a first heat exchanger, a second heat exchanger, a means for sending the mixture to be cooled to the first heat exchanger, a means for sending the cooled mixture to be partially condensed to at least the second heat exchanger which is a bottom reboiler of the second column, a means for sending to an intermediate level of the first column at least one portion of the liquid produced by the partial condensation or a liquid derived from this liquid by at least one step of partial condensation, only the mixture and bottom liquid from the second column exchanging heat in the second heat exchanger, a means for sending the vaporized bottom liquid back to the second column, a means for withdrawing a hydrogen-enriched overhead gas from the first column, a means for sending the overhead gas to be reheated to the first heat exchanger, a means for withdrawing a liquid enriched in carbon monoxide and in methane from the bottom of the first column, a means for sending the liquid enriched in carbon monoxide and in methane to at least one intermediate level of the second column, a means for withdrawing a methane-rich liquid from the bottom of the second column, a means for withdrawing a fluid rich in carbon monoxide from the top of the second column and a refrigeration cycle to produce refrigeration to keep the process cold. 